I have a detached garage with no HVAC system. It is brick with with a flat roof. No water leaks are present but I worry about the retained humidity in my Targas house. It is not totally sealed up as the wood soffit has gaps which allow air and humidy to enter and escape.

I do not think that these gaps in the wood soffit are large enough to circulate enough air. I am thinking about adding large vents every 10 feet or so similar to interior register vents, plus a turbine system vent on the roof.

The reason I am worried is because I found some surface rust in the boot this weekend when I pulled out the carpet. Not sure if its new or not but it must be getting worse since I just noticed it and have had the carpet out before.

What is the least expensive moisture absorbing compound I can put in the boot, interior, garage?

I live on the Washington coast about 300 yards from the ocean, the Pacific Northwest is noted for being very wet and damp.

I store my car in a heated area. Here, it is the best way to stop rust and corrosion while in storage. Desiccants work but they are hard to monitor and need to be dryed out. Heat is what works best as far as I am concerned. I store "Iris" at about 60 degrees unless I'm out in the shop, then we bump it up some.

As metal things heat and cool they attract moisture, by keeping an even temp and some air circulation mositure deposits are held down. We never, I mean never get hot humid days that breed rust.

I ventilated my garage with roof vents. Air goes under my garage door (the slab is sinking away from the door at the corners) so there is enough convection flow to quickly remove any moisture.
I keep desiccant bags in the interior and the trunk and a plastic vapor barrier under the car. This does nothing for high ambient humidity, however.

A heated garage is the ultimate but if the garage is shared with a daily driver there will still be humidity "spikes" when the snowy or wet daily car is put into the garage. I'm in the process of looking for another house and a 3 car or seperate, detached garage is a must-have.